Hello "Lord Muck,"
This forum sometimes has a low level of activity, so you may not get many responses. I will give you my 2 cents. Choosing a digital camera these days can be really overwhelming and confusing. First of all, before you decide on Nikon and DSLR, you could consider Canon or other types of Interchangeable Lens cameras (ILC's), i.e., mirrorless. But I'm not going to go there. Traditional DSLR's are still a great choice for nature photography, especially if you want to use telephoto for wildlife/birds, and although I often "bad mouth" Nikon, I still think they are just as good as Canon, especially now with the new D500.
I tend to be long-winded, so let me first point you at a very recent article by one of the Nikon photography writers I have read for a long time, Thom Hogan:
http://www.dslrbodies.com/newsviews/consumer-pro-dx-or-fx.html
Also the "
Photography Life" web site has a wealth of free information under the "Photography Tips" menu. I think every essay is dated at the top, so some of the information may be out of date, but I think most of it is still good. Keep in mind that the guy who started this site uses Nikon gear and the site tends to focus on Nikon gear.
I agree with Thom Hogan that with Nikon DSLR's, really it comes down to 2 basic decisions: (1) consumer or pro build, and (2) DX or FX sensor size. You're already focusing on the more affordable consumer grade choices (D7200 and D750), and I would not steer you away from those. Both are great cameras and both of these have been out for a while and are "proven performers," and they are now less expensive than when they were brand-new models.
As for your original question and DX vs. FX, assuming you have already decided to get a consumer level body (i.e., not the new D500 or D810), then your current top choices are the 2 cameras you mentioned, the D750 (FX) or the D7200 (DX). If bird photography is an important priority to you, I would definitely choose the D7200. The main reason is that it is very hard to get close enough to birds to fill the whole FX frame, especially with smaller birds, so you would end up cropping your D750 photos down to DX size or smaller anyway, and the final cropped image from the D750 would have less detail/resolution than the D7200. Also the money you save by getting the D7200 can be put into your
lens (unless you already have a lens). The D750 might be a better choice if you really want to mostly do landscape photography. As long as you fill the whole frame with your subject, the D750 will give you a little better image quality than the D7200. There are more and better wide angle lens choices for the D750 for landscapes and interiors, but there are still some decent lens options that allow you to get ultra-wide with DX.
Good luck to you!
Dave
